1. Field of the Invention
The invention relates to a process for continuously preparing silicone resins having a low content of alkoxy groups.
2. Background Art
The preparation of silicone resins has been known for some time. Silicone resins are preferably prepared by hydrolysis and condensation of chloro- or alkoxysilanes by means of acid or base catalysis. As an example of an alkoxysilane hydrolysis, reference is made to DE-B 1302773. Compared with chlorosilane hydrolyses, the alkoxysilane hydrolysis reaction is more moderate and are therefore easier to control. However, a disadvantage is a higher cost, since alkoxysilanes are first prepared from the corresponding chlorosilanes.
In addition to the hydrolysis of alkoxysilanes, the hydrolysis of chlorosilanes in particular has become commonplace, since a preceding alkoxylation is not required to prepare alkoxysilanes. Chlorosilane hydrolyses have belonged to the repertoire of silicone resin preparation for some time. As examples of batchwise chlorosilane hydrolyses, reference may be made to GB 1192506, DE 953661 C, DE 854708 C and DE 2415331 A. A disadvantage of these processes is the need for high dilution during the reaction, the reason for which is the high reactivity of the chlorosilanes. A further disadvantage of these processes is the large water excess, which leads to phase separation at the end of the reaction, in the course of which the HCl released is absorbed in the aqueous phase or even neutralized. Viewed on the industrial scale, this firstly results in a large amount of waste which requires disposal, and secondly, the HCl is lost, since it is not economically recoverable.
As examples of a continuous chlorosilane hydrolysis, reference may be made to U.S. Pat. No. 3,489,782 and DE 954198 C. In these references also, a large excess of water is needed, which leads to the wastewater problems mentioned above and the inability to economically recover HCl.
In addition to the classical batchwise and continuous synthesis methods described, technologies using at least one column have become established. These column processes enable a distinctly improved reaction. Thus, no wastewater phases are generated and the HCl released can be recovered as gas at the top of the column. Corresponding processes are described in U.S. Pat. No. 3,792,071 A, U.S. Pat. No. 4,298,753 A, U.S. Pat. No. 6,767,982 B2, U.S. Pat. No. 5,223,636 A1 and U.S. Pat. No. 4,209,454 A1. Even though these processes, compared with batch or loop technology, have been found to be a distinct improvement, they nevertheless have clear limitations which become obvious even in the examples of the documents mentioned. In all processes, only a low degree of condensation of the silicone resins is achieved, so that exclusively alkoxy-rich silicone resins are obtained in all cases. Silicone resins with a high degree of condensation and thus low alkoxy content, for example solid resins as obtained by means of classical batchwise or continuous synthesis methods not containing columns, have not been preparable by these processes to date. However, these products constitute an important constituent of the silicone resin market.